2-path hydromechanical transmission system and method of producing the same

ABSTRACT

A two-path hydromechanical transmission system used in a crawler-type vehicle and a method of controlling this transmission system. Excellent maneuverability and high degree of running course steadiness at low tramming speed, as well as high power transmission efficiency at high tramming speed, are obtained by the use of hydraulic pumps and motors of capacities smaller than those of known systems. The transmission system has a power transmission line including independent left and right hydraulic pumps 2, 3 driven by an engine 1 mounted on a vehicle and motors 4, 5 and reduction planetary gear trains 200, 250 having reduction gear trains 100, 150 for transmitting the output torques of said hydraulic motors to left and right driving wheels. The transmission system also has an additional power transmission line including a reduction gear device 300 for distributing the output torque of said engine 1 and said reduction planetary gear trains 200, 250 for transmitting the output torques of said reduction gear device to said left and right driving wheels. This transmission system is controlled in different modes for tramming at low and high tramming speeds.

TECHNICAL FIELD

The present invention relates to a speed-changing/steering system whichis used on a vehicle and which has both linear speed-changing andsteering functions, as well as to a method of controlling such a system.More particularly, the present invention is concerned with a 2-pathhydromechanical transmission system for use in crawler-type vehicles andalso to a method of controlling such a transmission system.

BACKGROUND ART

FIG. 6 shows a conventional power transmission system which is used oncrawler-type vehicles such as bulldozers and which is capable ofperforming both linear speed change and steering. This system, known as2-path hydrostatic transmission (referred to as 2-path HST, hereinafter)has, for the left and right crawler driving wheels 6, 7, hydraulic pumps2, 3 driven by an engine 1, hydraulic motors 4, 5 and reduction devices6a, 6b.

The power transmission system having such a 2-path HST, however, suffersfrom the following disadvantages due to the fact that all of the torqueis transmitted through the HST.

(1) Power transmission efficiency is low although the operability issuperior.

(2) This type of power transmission system is difficult to apply tolarge-size vehicles because hydraulic pumps and hydraulic motors capableof transmitting large power are not readily available are or, ifavailable, very expensive.

(3) Straight running performance of the vehicle is affected byefficiencies of the hydraulic pumps and motors.

(4) A complicated control is required for keeping the running course ofthe vehicle when the control is to be done through the control ofoperation of the hydraulic pumps and motors.

Accordingly, an object of the present invention is to provide a 2-pathhydromechanical transmission (referred to as 2-path HMT, hereinafter)which employs an HST incorporating hydraulic pumps and motors of smallcapacities and which exhibit high efficiency with simple constructionand ease of maneuver, as well as a control method for controlling such a2-path HMT, thereby overcoming the above-described problems of the priorart.

SUMMARY OF THE INVENTION

To this end, according to the present invention, there is provided a2-path hydromechanical transmission system, comprising: a powertransmission line including independent left and right hydraulic pumpsdriven by an engine mounted on a vehicle and motors associated with thehydraulic pumps, and reduction planetary gear trains having reductiongear trains and differential planetary gear devices for transmitting theoutput torques of the hydraulic motors to left and right driving wheelsof the vehicle; and an additional power transmission line including areduction gear device having a bevel gear for distributing the outputtorque of the engine and forward and reverse 2nd speed clutches to whichthe output torque of the engine is distributed, and the reductionplanetary gear trains for transmitting the output torques of thereduction gear device to the left and right driving wheels.

Left and right drive shafts to which the reduction gear trains and thereduction planetary gear trains are secured are connected to each otherthrough a coupling device having a center clutch.

When the vehicle trams forward at a low speed, an engine mounted on thevehicle drives left and right hydraulic pumps to actuate hydraulicmotors, the output torques of the hydraulic motors being transmitted toleft and right driving wheels via reduction gear trains and othercomponents, whereas, when the vehicle runs in the reverse direction, theleft and right hydraulic motors are reversed.

When the vehicle is steered to the left or right, the connection betweenleft and right drive shafts to which the reduction gear trains and othercomponents are secured is dismissed by disengagement of a center clutchin the coupling device interconnecting the drive shafts, and theoperation speeds of the left and right hydraulic motors are made todiffer to cause a difference in the rotation speed between the left andright driving wheels, thereby causing the vehicle to be steered.

When the vehicle trams forward at a high speed, a forward 2nd-speedclutch and the center clutch are engaged, whereas, when the vehicletrams in the reverse direction at a high speed, a reverse 2nd-speedclutch and the center clutch are engaged, and the output torque of anengine mounted on the vehicle is mechanically transmitted to planetarycarriers of differential planetary gear devices through a reductiondevice including a bevel gear which distributes the engine output torqueand forward/reverse 2nd-speed clutches to which the engine output torqueis distributed by the bevel gear, and at the same time, independent leftand right hydraulic pumps also are driven by the engine so as to driveassociated hydraulic motors the output torques of which are transmittedto sun gears of the differential planetary gear devices via reductiongear trains, so that the driving power transmitted mechanically and thedriving power transmitted hydraulically are added together at thedifferential planetary gear devices, whereby the sums of themechanically transmitted power and hydraulically transmitted power aretransmitted to the left and right driving wheels.

Furthermore, according to the method of the invention, the trammingspeed of the vehicle is controlled by increasing and decreasing rotationspeeds of hydraulic motors driven by an engine mounted on the vehiclewhen the tramming speed is low, whereas, when the vehicle is acceleratedduring tramming at a high speed, a control is conducted such that therotation speeds of the hydraulic motors is progressively decreased tozero and then the hydraulic motors are reversed and increase theirrotation speeds in the reverse direction.

Thus, according to the invention, different modes of transmission ofengine output torque to the driving wheels are selectively usedaccording to the tramming speed: namely, a low-speed mode in which theengine output torque is transmitted to the driving wheels through an HSTsystem, and a high-speed mode in which the engine output torque istransmitted to the driving wheels both through the HST system and an HMTsystem which transmits the torque mechanically. In addition, left andright drive shafts for transmitting the torques to the left and rightdriving wheels are connected to each other at the center of the vehicle,in order to obtain a high level of steadiness or course-holdingcharacteristic during straight running. It is therefore possible toobtain superior maneuverability and high degree of straight coursesteadiness during tramming at low speed, as well as high powertransmission efficiency at high tramming speed, by employing hydraulicpumps and hydraulic motors of reduced capacities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 2 and 3 are illustrations of the paths of power transmission in2-path HMT system embodying the present invention;

FIG. 4 is a chart illustrating the relationship between rotation speedof hydraulic motor and vehicle speed;

FIG. 5 is a chart illustrating the relationship between the vehiclespeed and hydraulic pressure; and

FIG. 6 is an illustration of the paths of power transmission in a known2-path HST system.

THE BEST MODE FOR CARRYING OUT THE INVENTION

A detailed description will be given of the 2-path HMT system embodyingthe present invention, as well as a method of controlling the same.

Referring to FIG. 1, a 2-path HMT system has two power transmissionlines: namely, a first power transmission line in which output torque ofan engine 1 is transmitted to left and right drive wheels 6, 7, throughindependent transmission lines including hydraulic pumps 2, 3, hydraulicmotors 4, 5, and reduction planetary gear trains 200, 250 includingreduction gear trains 100, 150 and differential planetary gears 101,151; and a second power transmission line in which the output torque ofthe engine 1 is transmitted to and divided by a reduction device 300having a bevel gear 28 and forward/backward 2-speed clutches 37, 39, thedivided torque being then transmitted to the left and right drivingwheels 6, 7 through the above-mentioned reduction planetary gear trains200, 250.

Referring first to the above-mentioned first power transmission line, aleft hydraulic pump 2 and a right hydraulic pump 3 are driven by a leftPTO driven gear 9 and a right PTO driven gear 10 which mesh with a PTOdrive gear 8 fixed to the output shaft of the engine 1. These hydraulicpumps 2 and 3 are connected, through hydraulic lines, to left and righthydraulic motors 4 and 5 which respectively drive left and right motorgears 11 and 12, respectively.

A left reduction gear train 100 is composed of the left motor gear 11and a left sun gear 14a which idles on a left drive shaft 13 and whichmeshes with the left motor gear 11. The left reduction planetary geartrain 200 includes: a differential planetary gear device 101 including aleft sun gear 14, a left planetary gear 15, a left planet carrier 16 anda left ring gear 17; a left final drive gear 18 directly connected tothe left ring gear 17; and a left final driven gear 19. The reductiongear train 150 and the reduction planetary gear train 250 for the rightdriving wheel have constructions similar to those for the left drivingwheel. Namely, the reduction gear train 150 for the right driving wheelincludes the right motor gear 12 and a right sun gear 21a which idles ona right drive shaft 20 and which meshes with the right motor gear 12.The reduction planetary gear train 250 for the right driving wheelincludes: a differential planetary gear device 151 composed of a rightsun gear 21, a right planetary gear 22, a right planetary carrier 23 anda right ring gear 24; a right final drive gear 25 directly connected tothe right ring gear 24; and a right final driven gear 26.

The second line of power transmission will now be described. The outputshaft of the engine 1 is connected through a propeller shaft 27 to abevel pinion 28 meshing with a bevel gear 29 which is fixed to a bevelshaft 32 together with forward 2nd (referred to as F2) drive gear 30 anda reverse 2nd (referred to as R2) drive gear 31. A forward gear train iscomposed of the F2 drive gear 30, an idler gear 33 meshing with the F2drive gear 30 and an F2 driven gear 34 which idles on the left driveshaft 13 and meshes with the idle gear 33. A reverse gear train iscomposed of the R2 drive gear 31 and an R2 driven gear 35 which idles onthe right drive shaft 20 and meshes with the R2 drive gear 31.

The left drive shaft 13 and the right drive shaft 20 are coupled to eachother through a coupling device 400 having a center clutch 36 and otherparts.

The F2 driven gear 34, left sun gear 14, R2 driven gear 35 and the rightsun gear 21 are respectively associated with an F2 clutch 37, a left 1stspeed clutch 38, an R2 clutch 39 and a right 1st-speed clutch 40.

The operation of the described system is as follows.

When the vehicle trams at a low speed, e.g., at 4.2 Km/h or lower speedas viewed in FIG. 5, the left 1st speed clutch 38 and the right 1stspeed clutch 40 are engaged, as viewed in FIG. 2, while the F2 clutch 37and the R2 clutch 39 are disengaged. In this state, the system functionsas the HST. Consequently, the output power of the engine 1 istransmitted from the left and right hydraulic pumps 2, 3 which arecontrolled to maintain a constant value of the product of the deliverypressure P and the displacement V to the hydraulic motors 4, 5 andfurther to the left and right sun gears 14, 21 through the left andright motor gears 11, 12. In this state, the differential planetary geardevices 101 and 151 are in direct-connection state so that the rotationsof the sun gears are transmitted to the planet carriers to rotate themat the same speed as the sun gears and further to the ring gears torotate them at the same speed as the planet carriers, whereby the leftand right final drive gears 18 and 25 are driven. Consequently, thespeed of the vehicle is increased in proportion to the rotation speed ofthe motor gears of the hydraulic motors as shown by AB (forward) and AE(reverse) in FIG. 4. In order to maintain the course of the vehiclerunning straight, the center clutch 36 is engaged to equalize the speedsof the left and right driving wheels. The power transmission pathbetween the propeller shaft 27 and the F2 and R2 driven gears 34, 35 viathe bevel gear 29 only idles and does not make any contribution to thetransmission of the power.

When the vehicle is to be steered to the left or right, the centerclutch 36 is disengaged to allow the left and right drive shafts 13, 20to rotate independently, and the rates of supply of the hydraulic fluidto the left and right hydraulic pumps 2, 3 are suitably varied to causea change in the speed between the hydraulic motors 4 and 5, therebyeffecting a gentle turn, quick turn or ultra-quick turn of the vehicle,while maximizing the traction power of the vehicle during turning.

For a high-speed forward tramming, e.g., forward tramming at a speedhigher than 4.2 Km/h as viewed in FIG. 5, the left and right 1st speedclutches 38, 40 are disengaged and the center clutch 36 and the F2clutch 37 are engaged, as viewed in FIG. 3. In this state, the systemperforms the function of HMT so that the output torque of the engine istransmitted from the propeller shaft 27 to the left and right driveshafts 13, 20 through the bevel gear 29, F2 drive gear 30, idle gear 33and the F2 driven gear 34, whereby the planet carriers 16, 23 of thedifferential planetary gear devices 101 and 151 are mechanically driven.

The power transmitted from the left and right hydraulic pumps 2, 3 tothe motor gears 11, 12 through the hydraulic motors 4, 5 drive the leftand right sun gears 14, 21 so that the ring gears 17, 24 of thedifferential planetary gear devices 101, 151 drive the left and rightfinal drive gears 18, 25 by the sum of the mechanical power derived fromthe planet carriers 16, 23 and the hydraulic power transmitted throughthe sun gears 14, 21. When the vehicle is to be steered to the left orright during high speed tramming, a difference is produced between theoperation speeds of the left and right hydraulic motors 4, 5 bydifferentiating the rates of supply of the hydraulic fluid from the leftand right hydraulic pumps 2, 3.

For a high-speed reversing of the vehicle, the F2 clutch 37 isdisengaged and the R2 clutch 39 is engaged so that the mechanical poweris transmitted from the bevel gear 29 to the R2 driven gear 35 throughthe R2 drive gear 31. Operations or other parts are the same as those inforward high-speed tramming.

The vehicle speeds at high speed ranges vary along the curves BCD(forward) and the curve EFG (reverse) shown in FIG. 4. Namely, areduction in the operation speed of the hydraulic motors 4, 5 causes areduction in the rotation speeds of the sun gears 14, 21 but the vehiclespeed increases because the rotation speeds of the ring gears 17, 24increase in inverse proportion to the rotation speeds of the sun gears14, 21. The speed of the hydraulic motors 4, 5 is null at the point Cduring forward tramming and the hydraulic motors 4, 5 start to reverseas the point C is exceeded. The speed of the hydraulic motors is thenprogressively increased. During reversing, hydraulic motors 4, 5 arereversed and the speed of these hydraulic motors becomes null at thepoint F and, thereafter, the hydraulic motors 4, 5 start to operateforward and progressively increase their speed.

As will be understood from the foregoing description, according to theinvention, the vehicle is driven through HST in the 1st-speed range inwhich delicate control is required. It is therefore possible to obtaingood maneuverability equivalent to that of conventional pure HST drivevehicle. Furthermore, since the left and right output shafts are drivencommonly through the coupling device, a high steadiness or courseholding characteristic can be attained without being affected by theefficiencies of the hydraulic pumps and motors. In addition, the controlis facilitated because there is no need for the control of thedisplacements of the hydraulic pumps and the hydraulic motors. Moreover,efficiency can be increased by about 10% or more as compared with thepure HST type drive system in which all the power is transmitted throughan HST system alone, by virtue of the use of the HMT system in which thedriving power is transmitted both through the HST system and themechanical transmission system in the high speed region where a largedriving power is required. It is also to be pointed out that the presentinvention enables the capacities of the hydraulic pump and the hydraulicmotor to be reduced to about half those in the known system thanks tothe use of rotation of the differential planet gear device, thusoffering an advantage also in the aspect of the cost. More particularly,according to the invention, it is possible to obtain an inexpensivepower transmission system which can realize a high tramming speed of 8.5Km/h or so at the maximum, by using, in combination with the mechanicaldriving system, hydraulic pumps and hydraulic motors of capacities whichcan provide, when used in a pure HST driving system, only a low trammingspeed of 4 Km/h or so at the maximum.

In the described embodiment of the invention, each hydraulic pump isconnected to the associated hydraulic motor through one hydraulic line.It will be clear, however, that the hydraulic pump and the hydraulicmotor are actually connected to form an open circuit, semi-open circuitor a closed circuit incorporating a change-over valve or a control valveto enable the control in the same manner a that in known systems.

INDUSTRIAL APPLICABILITY

The 2-path HMT system of the present invention, as well as the controlmethod of the same, can effectively used as a speed-change/steeringsystem of crawler-type vehicles.

I claim:
 1. A 2-path hydromechanical transmission system, suitable foruse with a vehicle having a left driven wheel and a right driven wheel,said system comprising:an engine; a first power transmission lineincluding a left hydraulic pump, a right hydraulic pump, said left andright hydraulic pumps being independent from each other, said left andright hydraulic pumps being driven by said engine, a left hydraulicmotor associated with said left hydraulic pump so as to be driven by theoutput of said left hydraulic pump, a right hydraulic motor associatedwith said right hydraulic pump so as to be driven by the output of saidright hydraulic pump, a left gear train for transmitting the outputtorque of said left hydraulic motor to a left drive wheel, a right geartrain for transmitting the output torque of said right hydraulic motorto a right drive wheel, to thereby provide a driving force from saidfirst power transmission line to said left and right drive wheels; aleft drive shaft associated with said left gear train, a right driveshaft associated with said right gear train; a coupling device having acenter clutch for selectively coupling said left drive shaft and saidright drive shaft so that said left drive shaft and said right driveshaft are driven commonly through said coupling device when said centerclutch is engaged; a second power transmission line including a forwardhigh speed clutch, a reverse high speed clutch, a reduction gear deviceincluding a bevel gear for distributing output torque of said enginethrough a selected one of said forward high speed clutch and saidreverse high speed clutch and through said left drive shaft and saidright drive shaft to said left and right gear trains to thereby providea driving force from said second power transmission line to said leftand right drive wheels; whereby said first power transmission line canprovide the power for driving the vehicle at a low speed, and said firstand second power transmission lines can, in combination, provide thepower for driving the vehicle at a high speed.
 2. A 2-pathhydromechanical transmission system according to claim 1, wherein saidleft gear train comprises a left reduction gear train and a leftreduction planetary gear train, with said left reduction gear trainbeing connected between said left hydraulic motor and said leftreduction planetary gear train; and wherein said right gear traincomprises a right reduction gear train and a right reduction planetarygear train, with said right reduction gear train being connected betweensaid right hydraulic motor and said right reduction planetary geartrain.
 3. A 2-path hydromechanical transmission system according toclaim 2, wherein said left reduction gear train comprises a left motorgear which is driven by said left hydraulic motor, and a first sun gear,wherein said first sun gear idles on said left drive shaft and mesheswith said left motor gear; and wherein said right reduction gear traincomprises a right motor gear which is driven by said right hydraulicmotor, and a second sun gear, wherein said second sun gear idles on saidright drive shaft and meshes with said right motor gear.
 4. A 2-pathhydromechanical transmission system according to claim 3, wherein saidleft reduction planetary gear train comprises a left differentialplanetary gear device and a left drive gear directly connected to theoutput of said left differential planetary gear device; and wherein saidright reduction planetary gear train comprises a right differentialplanetary gear device and a right drive gear directly connected to theoutput of said right differential planetary gear device.
 5. A 2-pathhydromechanical transmission system according to claim 3, wherein saidleft reduction planetary gear train comprises a left planetary gear, athird sun gear, said third sun gear being connected to said first sungear and meshing with said left planetary gear, a left planet carrier, aleft ring gear, and a left drive gear directly connected to said leftring gear; and wherein said right reduction planetary gear traincomprises a right planetary gear, a fourth sun gear, said fourth sungear being connected to said second sun gear and meshing with said rightplanetary gear, a right planet carrier, a right ring gear, and a rightdrive gear directly connected to said right ring gear.
 6. A 2-pathhydromechanical transmission system according to claim 5 wherein saidleft planet carrier is connected to said left drive shaft, wherein saidright planet carrier is connected to said right drive shaft, wherein aleft low speed clutch connects said first sun gear to said left driveshaft, and wherein a right low speed clutch connects said second sungear to said right drive shaft.
 7. A 2-path hydromechanical transmissionsystem according to claim 6 wherein said forward high speed clutchconnects said reduction gear device to one of said left drive shaft andsaid right drive shaft, and wherein said reverse high speed clutchconnects said reduction gear device to the other of said left driveshaft and said right drive shaft.
 8. A vehicle having a left drivewheel, a right drive wheel, and a 2-path hydromechanical transmissionsystem in accordance with claim
 7. 9. A vehicle having a left drivewheel, a right drive wheel, and a 2-path hydromechanical transmissionsystem in accordance with claim
 1. 10. A method for controlling thespeed and direction of motion of a vehicle having a left drive wheel, aright drive wheel, and a 2-path hydromechanical transmission systemmounted on said vehicle, said 2-path hydromechanical transmission systemhaving an engine, a first power transmission line, and a second powertransmission line; said first power transmission line including a lefthydraulic circuit and a right hydraulic circuit; said left hydrauliccircuit including a left hydraulic pump which is driven by said engine,a left hydraulic motor which is driven by said left hydraulic pump, aleft gear train for transmitting the output torque of said lefthydraulic motor to said left drive wheel, and a left drive shaftassociated with said left gear train; said right hydraulic circuitincluding a right hydraulic pump which is driven by said engine, a righthydraulic motor which is driven by said right hydraulic pump, a rightgear train for transmitting the output torque of said right hydraulicmotor to said right drive wheel, and a right drive shaft associated withsaid right gear train; said left and right hydraulic pumps beingindependent from each other; said second power transmission lineincluding a forward high speed clutch, a reverse high speed clutch, areduction gear device for distributing output torque of said enginethrough a selected one of said forward high speed clutch and saidreverse high speed clutch and through said left drive shaft and saidright drive shaft to said left and right gear trains to thereby providea driving force from said second power transmission line to said leftand right drive wheels; wherein said method comprises:utilizing onlysaid first power transmission line to drive said left drive wheel andsaid right drive wheel during a low speed mode, coupling said left driveshaft and said right drive shaft so that said left drive shaft and saidright drive shaft are commonly driven when it is desired that saidvehicle move in a straight line during said low speed mode, anduncoupling said left drive shaft and said right drive shaft and drivingsaid left hydraulic motor and said right hydraulic motor at differentspeeds while said left drive shaft and said right drive shaft are thusuncoupled, when it is desired that said vehicle turn during said lowspeed mode.
 11. A method in accordance with claim 10 furthercomprising:utilizing both said first power transmission line and saidsecond power transmission line to drive said left drive wheel and saidright drive wheel during a high speed mode.
 12. A method in accordancewith claim 11 wherein said reduction gear device is connected throughsaid reverse high speed clutch to one of said left and right driveshafts when it is desired that said vehicle move backwardly during ahigh speed mode.
 13. A method in accordance with claim 11 wherein saidreduction gear device is connected through said forward high speedclutch to one of said left and right drive shafts when it is desiredthat said vehicle move forwardly during a high speed mode.
 14. A methodin accordance with claim 13 wherein said reduction gear device isconnected through said reverse high speed clutch to one of said left andright drive shafts when it is desired that said vehicle move backwardlyduring a high speed mode.
 15. A method in accordance with claim 14further comprising:coupling said left drive shaft and said right driveshaft during said high speed mode so that said left drive shaft and saidright drive shaft are commonly driven.
 16. A method in accordance withclaim 15 further comprising:driving said left hydraulic motor and saidright hydraulic motor at different speeds while said left drive shaftand said right drive shaft are coupled, when it is desired that saidvehicle turn during said high speed mode.
 17. A method in accordancewith claim 11 further comprising:coupling said left drive shaft and saidright drive shaft during said high speed mode so that said left driveshaft and said right drive shaft are commonly driven.
 18. A method inaccordance with claim 11 further comprising driving said left hydraulicmotor and said right hydraulic motor at different speeds while said leftdrive shaft and said right drive shaft are coupled, when it is desiredthat said vehicle turn during said high speed mode.
 19. A method inaccordance with claim 11 wherein said reduction gear device is connectedthrough said reverse high speed clutch to one of said left and rightdrive shafts when it is desired that said vehicle move backwardly duringa high speed mode.